This invention relates generally to wind turbines, and more particularly to methods and apparatus for reducing peak loads, deflections, and/or rotational speed of wind turbines such as those that occur in a changing wind environment during normal operation.
Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted to a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators that may be rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.
It has been observed that peak loads and/or rotational speeds of pitch regulated wind turbines often occur as a result of wind speed dropping suddenly to a level in which the pitch controller demands a rapid pitch toward minimum pitch followed by a large increase in wind speed. In the worst cases observed, the pitch controller cannot keep up with the wind change and large blade deflections, loads, and rotational speeds arise. In at least one known wind turbine configuration, a time lag is provided to limit the minimum pitch of the pitch control system. However, the time lag is based upon the power output of the turbine and is not always active to limit the minimum pitch.